1. Field of the Invention
An object of the invention is a two-band transmitter/receiver with a dual radiation device. It can be applied more particularly in the field of mobile telephones. In this field, the transmitters/receivers concerned are quite simply mobile telephones that are transmitters/receivers and are two-band devices at least because they are capable of adapting to one standard or another (or even to a third or fourth standard). The standards concerned are especially the so-called GSM standard where the transmission/reception operations are carried out in a 900 MHz band, the DCS standard where the transmissions/reception operations are carried out in the 1800 MHz band, the PCS standard where they are carried out in a 1900 MHz band, or again the UMTS standard where they are carried out in the 2200 MHz band.
The invention is essentially useful when instruments of this kind are provided with two means of radiation. For this purpose, there is a known first means of radiation which is the antenna of a mobile telephone. However, these mobile telephones are in most cases provided in their socket with a connector capable of broadcasting transmissions (and also being used for reception) by being connected to an aerial other than that of the mobile telephone, for example that of a vehicle in which this mobile telephone is placed.
2. Description of the Prior Art
The problems encountered in this type of equipment lie chiefly in an antenna selector switch used to broadcast the power radiated in transmission either in the antenna proper of the mobile telephone or by means of the socket connector of this mobile telephone in an external aerial. Indeed, these antenna selector switches are made up of integrated circuits, especially MOS type transistor circuits. These MOS transistors, depending on whether they are on or off, let through power to be broadcast to one aerial or another.
It can furthermore be seen that the power values to be dissipated vary from one standard to another. Indeed, these power values are related to the conditions of propagation, according to the frequency ranges. Thus, it is accepted that, according to the GSM standard, it is possible to transmit to two watts of radioelectric energy. By contrast, in the DCS and PCS standards as well as in the UMTS standards, the transmission power is limited to one watt.
Just as the passage from a power value of one watt through the transistors constituting the output selector switches does not raise any particular difficulty, so it can seen that a power value of two watts is greater than a borderline power of linear operation of the currently available selector switch transistors. These selector switch transistors then work in a non-linear zone. More specifically, the selection switching leads to a class B operation of these transistors. Under these conditions, these transistors distort the transmitted signals. The chief consequence of this is the sending of a second harmonic of the fundamental transmission frequency.
A second harmonic such as this has a first drawback: that of dispensing power unnecessarily since it will not be picked up. It furthermore has a second drawback that is far more troublesome: it forms a source of uncontrollable parasites in the bands of frequencies higher than that of the base signal. In practice, it is assumed that a GSM-DCS type output selector switch is a source of noise that causes trouble for the DCS, PCS or UMTS type networks. Naturally, the same phenomena of distortion occur for the latter networks, when the output selector switch permits transmission according to the last-named standards. However, the problem then is less troublesome firstly because the power emitted is then weaker and secondly because, to date, there are no users at frequencies twice the nominal frequencies of the last-named three ranges.
A simple solution used to resolve this problem could consist in setting up a filter at output of the selector switch. The aim of this filter would be to dampen the second harmonics emitted in a range. The presence of a filter of this kind would unfortunately be unacceptable because it would ipso facto attenuate transmission and reception in the other three ranges. Otherwise, it may be planned to make this filter itself switchable. However, this brings us back to the previous problem where the selector switch of this filter would itself induce parasites by distortion whose effects cannot be undone.
In GSM, DCS, PCS or other types of telephony, the instruments used are not duplex instruments. In fact, they are either transmitters or receivers in alternation. There are several transmission systems. These include the systems known as TDMA (time division multiple access), FDMA (frequency division multiple access), CDMA (coded division multiple access) or and combined systems using these three systems. The TDMA system works by time division while the other two systems work by the sharing of a frequency plane or a encoding system. However in these three systems, the machines are either in transmission mode or in reception mode: there are never in both modes at the same time. There is therefore an automatic alternation managed by a microprocessor of the mobile telephone that works according to instructions given to it by a base station and according to a desired protocol. According to this protocol, transmission phases follow reception phases in alternation, whether the phases are continuous in time or sporadic. The duplexer function thus achieved is then also obtained in the form of an alternating selector switch.
In two-band machines (especially GSM-DCS machines), finally there is a third type of selector switching: a band selector switch or a mode selector switch to go from one standard to the other.
There is no known total selector switch function. Given the complexity of the three types of cascaded selector switching functions, namely the antenna selector switch, the alternation selector switch and the mode selector switch, it has been discovered in the invention that it is simpler to place the antenna selector switch down-line with respect to the assembly. The number of transistors used for the total selector switch is then reduced to its minimum. This leads to a second harmonic rate that is as low as possible.
Thus, an object of the invention is a transmission/reception circuit comprising:
a first pair of reception channels respectively tuned to a frequency f0 and to a frequency f1 different from f0 and connected to two inputs of a first diplexer, a second pair of transmission channels respectively tuned to a frequency f0 and a frequency f1 and connected to two inputs of a second diplexer,
a first and second radiation circuit,
and a selector switch connected at input to the outputs of the two diplexers and at output to the two radiation circuits, to receive these two channels and switch over a transmission channel and a reception channel tuned to one and the same frequency on one of the radiation circuits.
Furthermore, in the transmission phases, which are the most critical from the viewpoint of the transmission of the second harmonic, to prevent the propagation of a second harmonic such as this, especially in the case of a GSM type use (in order not to create parasites in the DCS, PCS and UMTS bands), it is planned in the invention to obtain a non-switched filter. In the invention, the components of this non-switched filter are located partly after and partly before the mode selector switch. In a preferred variant, these components are placed on either side of the general selector switch. Thus, the reactive impedance part located before this selector switch gets combined with another reactive impedance part located before this selector switch on a transmission channel to attenuate the second harmonic.
When the use is switched to another mode, the signal sent no longer perceives anything on its path other than one of these two reactive impedance parts, the one located after the mode selector switch. Indeed, in the transmission on this other channel, there is no other reactive impedance or an impedance at a different value. Thus, this other signal is not dampened and can get freely propagated. This was the goal to be achieved.
It is also an object of the invention therefore to obtain a transmission circuit comprising a transmission channel for a carrier at a frequency f0, a transmission channel for a carrier at a frequency f1 different from f0, a radiation circuit and a selector switch to receive these two transmission channels and switch over one of these two transmission channels to the radiation circuit, wherein the transmission circuit comprises a first filter element located between the selector switch and the radiation circuit, this first filter element being tuned to a second filter element located in the transmission channel at the frequency f0 to prevent the transmission, through this frequency channel at the frequency f0, of frequency components at the frequency f1.